The present invention relates to a cold-stretchable, self-adhesive film composition.
The literature gives numerous examples of compositions, based on ethylene polymers, for the manufacture of self-adhesive plastic films. Typically, in a first stage, these compositions contain a low density polyethylene (obtained by a free radical polymerization process) and varying amounts of an ethylenepropylene copolymer of an elastomeric type, having a Mooney viscosity of between 15 and 80. Examples of such compositions are described in French Patent Nos. 2,368,416, 2,405,972, British Patent No. 2,038,845, and U.S. Pat. Nos. 4,110,414 and 4,123,417. Subsequently, other documents have described the need for a supplementary treatment by a corona discharge, as in French Patent No. 2,392,064, and the possibility of replacing the low density polyethylene by a linear ethylene-1-butene copolymer, as in European Patent No. 0,014,539. European Patent No. 0,011,930 has described the possibility of replacing the ethylene-propylene copolymer of an elastomeric type by a polyisobutylene and of combining this with a low density polyethylene or with an ethylenevinyl acetate copolymer.
Moreover, French Patent No. 2,277,852 (EXXON) describes a composition for the manufacture of a self-adhesive film, wherein the film is comprised of:
(a) from 85 to 99.8% by weight of a thermoplastic polyolefin, PA0 (b) from 0.1 to 10.00% by weight of a partial ester of a carboxylic acid and a polyol, and PA0 (c) either from 0.1 to 10% by weight of a hydrogenated hydrocarbon resin, or from 0.1 to 10% by weight of polybutene or of polyisobutene, with the proviso that the combined weights of (b) and (c) do not exceed 15% by weight. The patent states, on page 5, lines 32-34, that the preferred amount of constituent (c) is less than or equal to 3% by weight. PA0 (a) from 88 to 97% by weight of an ethylene-.alpha.-olefin copolymer, the .alpha.-olefin having from 3 to 10 carbon atoms and the copolymer having a density of between 0.905 and 0.940 g/cm.sup.3 and a melt index of between 0.1 and 2 dg/minute; and PA0 (b) from 3 to 12% by weight of a polymer selected from among polyisobutylene, atactic polypropylene, cis-polybutadiene and bromobutyl rubber.
Moreover, it is known that the films manufactured from low density polyethylene (obtained by a free radical polymerization process) exhibit properties, such as ease of stretching, cohesive force and mechanical properties, that have been inadequate and thus such films have not enjoyed major development in the cold-stretchable film technique. Cold-stretching is a packaging technique which consists of keeping several bundles of identical or different nature, but of relatively regular geometrical shape, grouped together by stretching a film, at ambient temperature, around these bundles. The film can be stretched either by the curtain machine technique or by the wrap-round technique.
In the curtain machine technique, the width of the film arranged vertically corresponds to the height of the contents. The ends of the film, stretched either directly or indirectly, are welded over the whole of this width. The welding is effected under tension so as to maintain the stretching.
The wrap-round technique can be sub-divided into two embodiments: the straight wrap-round technique and the spiral wrap-round technique. In the straight wrap-round technique, the width of the film in general corresponds to the height of the contents, and the unwound film is stretched between a braking cylinder and the contents themselves. In the spiral wrap-round technique, the width of the film is less than the height of the contents, and the contents rotate and stretch the film as in the preceding case. To cover the entire height of the contents, the film is subjected to an upward movement and hence describes a spiral. The stretching that takes place between the braking cylinder and the contents, or between the reel of film, which is itself braked, and the contents, depends on the speed of rotation of the contents and/or the degree of freedom imposed on the braking cylinder or on the reel. In the spiral wrap-round technique, currently the most widely used, complete automation requires that positioning of the ends of the film should be achieved. This problem is one of those to which the present invention provides a solution.
As has been seen from the various patents cited above, the plastics most commonly used in the cold-stretchable film technique have been low density polyethylene (obtained by a free radical polymerization process) and ethylene-vinyl acetate copolymers. These polymers, to which the addition of an ethylenepropylene copolymer of an elastomeric type imparts self-adhesive properties, exhibit characteristics which restrict the scope for development of the cold-stretchable film technique. Thus, in view of the speeds of stretching (of the order of 40 m/minute between the reel and the contents) usually employed in this technique, it is essential to restrict the degree of stretch of these polymers to about 30% in order not to run the risk of generating a stretching force close to the breaking strength of the polymer and hence so as not to cause continual breaks during production. The mechanical properties of the cold-stretched films of the prior art, however, were mediocre: an elongation at break of the order of 500%, a breaking strength of the order of 200 kg/cm.sup.2, and a tear strength of the order of 250 g, for a 50 .mu.m thick film.